Commitment to cell division depends on cells growing to a certain size, called the critical size. The main objective of this proposal is to find out why critical size must be achieved, and what it is about size that allows the cell cycle to go forward. Previously, we discovered that size is linked to the steady-state levels of a critical G1 cyclin called Cln3 in yeast, whose homolog in mammalian cells is cyclin D. It appears that Cln3 is titrated against the number of DNA-bound SBF transcription factor complexes in the cell, and the number of these is simply set by genomic sequence. That is, the amount of Cln3, which grows as the cells grow, is titrated against something fixed, the number of DNA binding sites for a transcription factor complex. This allows cells to measure critical size. In the first two Aims, we will confirm, extend, and explore this titration model, in S. cerevisiae, and also in other species, to establish the generality of this mechanism. In addition, we have found that size control mechanisms remain even when the Cln3 pathway is abolished. One of these additional size control mechanisms seems to apply only in slowly growing, carbon-limited cells, and that mechanism seems to involve a cellular measurement of the storage carbohydrates glycogen and trehalose. In the third Aim, we will explore this new mechanism, with particular attention to the possibility that metabolic control pathways, and cell cycle control pathways, are more directly connected that is generally appreciated. These studies could be related to the Warburg effect in cancer cells.